Hoisting devices, such as drawworks, need to satisfy multiple design criteria including maximum lift capacity and maximum speed of empty or near-empty loads. These two criteria are antagonistic under current design practices since high lift capacity requires relatively high reduction ratios while high hoisting speed requires low reduction ratios.
Compromise between the two requirements is often found in multi-speed hoisting device designs, where low-speed reduction ratios are used for slow lifting of heavy loads, while high-speed ratios are used for moving relatively light loads or empty loads at relatively high speeds.
This compromise approach, however, does not address the need for redundancy of hoisting capacity in case of a motor failure. Namely, if one motor of a multi-motor hoisting device fails, hoisting speed(s) will not be affected, but the hoisting capacity will be lowered proportionally. For example, with a two-motor hoisting device, if one motor fails, hoisting capacity would be halved. However, hoisting capacity should be prioritized over hoisting speed because the ability to lift a full load is more important than the ability to lift half of the load at full speed.
There is a need for hoisting devices with fail-safe lifting capacity, with optimal performance when all motors are fully operational, and with the ability to maintain full hoisting capacity (albeit with a loss of hoisting speed) in case of a motor failure.